I think the calorie requirement works out to 15 cals per pound of body weight. So you want 2025 per day as a minimum to maintain body weight or thereabouts. To gain weight you need more than this so set 2500 (maybe higher as you said you have a fast metabolism as I recall).

Just be careful. When you hit the target weight watch the calorie balance or you may find you put on more weight than you want!

There's a thread somewhere on here with some great milkshake drinks you can whip up at brekkie time to kick off the day with plenty of good stuff. Here's my old weight gain one for the record